Steaming and burning lava blocks floating to the surface - "Lava Balloons"
A similar phenomenon was observed during the submarine eruption off La Terceira Island (Azores) in January 1999. It was in a similar stage as the current El Hierro eruption. It did not breach the surface, nor reach Surtseyan activity, i.e. explosive magma-water interaction at shallow depths.
- Forjaz, V. H.; Rocha, F. M.; Medeiros, J. M.; Meneses, L. F. & Sousa, C. (2000) "Notícias sobre o Vulcão Oceânico da Serreta, Ilha Terceira dos Açores" Ed. OGVA
Lava produced by this submarine fissure eruption was basaltic, showing phenocrysts of olivine, pyroxene, and plagioclase in a highly vesicular groundmass. Analysis of the sample suggested that material rose from the sea bottom as hot lava balloons, lost gas from their interiors at the surface, and then sank. ..."
"Floating blocks were seen on the surface ... and some were collected. The scientific team attributed their seismic observations coupled with the floating blocks to the following mechanism:
The magma, being low in viscosity, moves very easily through the already opened fractures and was thought to escape without producing high seismic signals. Floating lava blocks could result from the detachment of pillow-lava edges followed by the ascent of blocks with sufficient gas content. It is also possible that hot, gas-rich lava fragments result from small submarine lava lakes or fountains.
A thin frozen skin of lava seals the gas cavity, and the block might then rise as a hot lava balloon. During ascent, the gas exsolves and nucleates inside the hot fragment while the blocks expand. Once at the surface the interaction between the hot blocks and the seawater produces white steam columns. At the same time, while cooling at the surface, the blocks crack slowly, lose their magmatic gas and sink. Sometimes when water enters inside the hot blocks, they blow up, violently throwing fragments several meters high."
from: Smithsonian / GVP monthly reports (Terceira)